Getting to Know MCDW

We’ve talked a lot about Modified Circumpolar Deep Water recently. It’s the water that our gliders are looking for, and our scientists think the nutrients it carries cause the great blooms of food during the Ross Sea’s summers. Let’s take a day to get to know it.

MCDW starts out in water 400 meters (1320 feet) or more below the surface of the Southern Ocean. It travels around and around Antarctica as part of an enormous current called the Antarctic Circumpolar Current (‘circumpolar’ just means ‘around the pole’). If you add up all the water in all the world’s rivers, the Antarctic Circumpolar Current carries about 100 times that total.

At the opening of the Ross Sea, some of that water gets pushed by the tides and jostled by the rotation of the Earth until it breaks off from the circumpolar current. It begins flowing into the Ross Sea, carrying with it characteristics of the water it came from, including a temperature of about 30°F to 32°F and a salinity value of 34.5 to 34.6. It also carries deep-water nutrients such as iron toward the shallower, sunlit waters of the Ross Sea.

When our scientists look at data from a CTD or glider, these characteristics make it easy to recognize MCDW. But temperature and salinity have a more important effect, too: they determine how dense the water is. And that determines where the MCDW moves. Read on through the slideshow to find out more, and for a chance to discover Modified Circumpolar Deep Water in your own data plot:

Now that you’ve been properly introduced to MCDW, see if you can find the signs of MCDW in this graph. It shows real data from a CTD that we put in the water earlier today. Take a look, and we’ll explain it below:

To find the telltale signs of MCDW, remember that it is

1. Not at the surface
2. Warmer than surrounding water
3. Contains less oxygen than surrounding water

The graph’s horizontal axis shows depth below the surface. The red line shows you the way that the temperature rose and fell as the CTD went down through the water. The blue line shows you how much oxygen was in the water.

From what you know about MCDW, you’re looking for a place where the temperature (red line) is high, and the oxygen (blue line) is low. If Dr. Kohut asked you how deep the MCDW was, what would you tell him? We’ll tell you the answer in Monday’s post.

(Many thanks to Dr. Phoebe Lam for making the graph and Dr. Bruce Huber for sharing his knowledge of ocean currents.)

About Hugh Powell

Hugh is a staff writer at the Cornell Lab of Ornithology and is on special assignment with the Rutgers University Institute of Marine and Coastal Sciences. He has previously written for the Woods Hole Oceanographic Institution.